1. Field of the Invention
The present invention relates to an optical receiver circuit, in particular, the invention related to an optical receiver that includes an avalanche photodiode (APD) to receiver an optical signal with a digital format and a PIN photodiode that receives another optical signal with an analog format.
2. Related Prior Art
A subscriber optical system, typically called as the Passive Optical Network (PON) system, has been practically introduced. One of configurations of the PON system transmits three optical signals, that is, two digital optical signals one of which is transmitted from the head end to the subscriber and has the wavelength of 1.49 μm and the other of which is transmitted from the subscriber to the head end and has the wavelength of 1.31 μm, and, in addition of these two signals, an analog signal from the head end to the subscriber and has the wavelength of 1.55 μm.
The optical network terminal (hereafter denoted as ONT) set in respective subscribers is necessary to install a laser diode to transmit the digital signal with the wavelength of 1.31 μm, a photodiode to receive the other digital signal with the wavelength of 1.49 μm, another photodiode to receive the analog optical signal with the wavelength of 1.55 μm, and a wavelength division multiplexing filter (hereafter denoted as WDM filter) to multiplex or to de-multiplex these three optical signals.
FIG. 1 shows a configuration of the typical GPON system. This system includes the optical line terminal 1 (hereafter denoted as PLT) that provides the video online terminal (hereafter denoted as video OLT) 1a and the basic band OLT 1b. The former OLT, the video OLT, 1a delivers the video signal in the analog form with the wavelength of 1.55 μm, while, the latter OLT, the basic band OLT, 1b transmits the digital signal with the wavelength of 1.49 μm and concurrently receives the other digital signal with the wavelength of 1.31 μm. The WDM filter 3a de-multiplexes the optical signal with the wavelength of 1.31 μm from the signal with the wavelength of 1.49 μm, while, the other WDM filter 3b multiplexes the signal with the wavelength of 1.55 μm from the signals with the wavelengths of 1.31 and 1.49 μm. Thus, the signals with the wavelengths of 1.49 μm and 1.55 μm are transmitted in the optical fiber 4 to the subscribers after being ramified by the coupler 5.
In respective ONTs 2, the received wavelength multiplexed optical signal is divided into two signals, one of which has the analog format with the wavelength of 1.55 μm and the other of which has the digital format with the wavelength of 1.49 μm by the WDM filter 6, and is received by respective PDs. Specifically, the analog signal with the wavelength of 1.55 μm is received by the video receiver 2a that installs the PIN-PD, while, the digital signal with the wavelength of 1.49 μm is received by the digital receiver 2b with an avalanche photodiode (hereafter denoted as APD). The ONT 2 also provides the digital transmitter 2c that transmits the other digital signal with the wavelength of 1.31 μm to the receiver in the basic band OLT 1b in the OLT 1.
The digital receiver 2b usually implements an APD with the carrier multiplication function to get high sensitivity. The multiplication factor of the APD increases by raising the bias voltage applied thereto. However, unnecessary higher bias voltage results in the increase of the noise to degrade the sensitivity, the saturation of the downstream circuit and sometimes causes the breakdown of the APD by the self-current. Contrary, a less multiplication factor also degrades the sensitivity of the APD. Because the multiplication factor of the APD depends not only on the bias voltage but on the temperature thereof, various techniques to set the optimum multiplication facto of the APD has been proposed.
A Japanese Patent Application published as JP-2000-244418A has disclosed an optical communication system where an optical receiver provides a PIN-PD in addition to an APD, a temperature sensor, a memory and so on. The PIN-PD receives the signal light which the APD also receives, while, the memory stores an adequate voltage applied to the APD in connection with the temperature. The system adjusts the bias voltage applied to the APD based on the output from the PIN-PD that corresponds to the optical input level to the APD and the adequate voltage stored in the memory. Another Japanese Patent Application published as JP-S59-160345A has disclosed an optical receiver circuit that provides a shunting transistor connected in parallel with the APD. This transistor may quickly shunt the current supplied from the power supply to the APD, which enhances the response of the optical receiver circuit.
Still further, a Japanese Patent Application published as JP-H05-343926A has disclosed an optical receiver circuit that provides a dumping resistor disposed between the bias supply and the APD. This dumping resistor may adjust the bias voltage directly applied to the APD by the current feedback function. A Japanese Patent Application published as JP-2005-304022A has disclosed an optical receiver with an APD whose bias voltage is supplied through the current mirror circuit. The receiver circuit disclosed therein adjusts the bias voltage to the APD so as to keep constant the average photocurrent generated in the APD and monitored through the current mirror circuit.
Prior receiver circuits described above have left various subjects. For instance, the optical receiver system is necessary to implement an additional PIN-PD to monitor the optical input level. To shunt the bias current by the shunting transistor connected in parallel to the APD increases the power dissipation of the receiver circuit. The shunted current only wastes the power. For the receiver circuit that provides the dumping resistor, the power consumption may increase by this dumping resistor when the optical input level becomes large and the APD generates relatively larger current.
The present invention is to provide an optical receiver circuit without any additional PD only for monitoring the optical input level to the APD, any circuits to compensate the temperature characteristic of the APD, and any increase the power consumption of the circuit around the APD.